This invention generally relates to systems and methods for detecting and analyzing compositions. More particularly, this invention is directed to a system and method for detecting, analyzing, and determining the extent of contaminant build up on components exposed to high temperatures, such as turbine components protected by coatings that are susceptible to damage by infiltration of contaminants in the operating environment of a turbomachine.
Hot section components of turbomachines, including gas turbines employed for power generation and propulsion, are often protected by a thermal barrier coating (TBC) to reduce the temperature of the underlying component substrate and thereby prolong the service life of the component. Ceramic materials and particularly yttria-stabilized zirconia (YSZ) are widely used as TBC materials because of their high temperature capability, low thermal conductivity, and relative ease of deposition by plasma spraying, flame spraying and physical vapor deposition (PVD) techniques. Plasma spraying processes such as air plasma spraying (APS) yield noncolumnar coatings characterized by a degree of inhomogeneity and porosity, and have the advantages of relatively low equipment costs and ease of application. TBC's employed in the highest temperature regions of turbomachines are often deposited by PVD, particularly electron-beam PVD (EBPVD), which yields a strain-tolerant columnar grain structure. Similar columnar microstructures with a degree of porosity can be produced using other atomic and molecular vapor processes.
To be effective, a TBC must strongly adhere to the component and remain adherent throughout many heating and cooling cycles. The latter requirement is particularly demanding due to the different coefficients of thermal expansion (CTE) between ceramic materials and the substrates they protect, which are typically superalloys, though ceramic matrix composite (CMC) materials are also used. An oxidation-resistant bond coat is often employed to promote adhesion and extend the service life of a TBC, as well as protect the underlying substrate from damage by oxidation and hot corrosion attack. Bond coats used on superalloy substrates are typically in the form of a diffusion aluminide coating or an overlay coating such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium, a rare earth element, or a reactive element). During the deposition of the ceramic TBC and subsequent exposures to high temperatures, such as during turbine operation, these bond coats form a tightly adherent alumina (Al2O3) layer or scale that adheres the TBC to the bond coat.
The service life of a TBC system is typically limited by a spallation event driven by bond coat oxidation, increased interfacial stresses, and the resulting thermal fatigue. In addition to oxide growth between the bond coat and TBC and CTE mismatch between the TBC (ceramic) and substrate (typically metallic), spallation can be promoted as a result of the TBC being contaminated with compounds present in the airflow of a turbomachine during its operation. Notable contaminants include varying mixtures of oxides such as calcia, magnesia, alumina and silica, whose presence becomes more prevalent when the ambient air contains particulates of dirt, volcanic ash, cement dust, and/or other materials containing these oxides. When present together at elevated temperatures, calcia, magnesia, alumina and silica can form a eutectic compound referred to herein as CMAS. CMAS has a relatively low melting temperature of about 1225° C. (and possibly less depending on its exact composition), such that during turbine operation the CMAS can melt and infiltrate the porosity within cooler subsurface regions of the TBC, where it resolidifies. As a result, during thermal cycling TBC spallation is likely to occur from the infiltrated and solidified CMAS interfering with the strain-tolerant nature of columnar TBC, particularly TBC deposited by PVD and APS due to the ability of the molten CMAS to penetrate their columnar and porous grain structures, respectively. Another detriment of CMAS is that the bond coat and substrate underlying the TBC are susceptible to corrosion attack by alkali deposits associated with the infiltration of CMAS. Finally, it should be noted that the likelihood of CMAS infiltration becomes greater as higher operating temperatures are used to increase the efficiency of turbomachines.
Various studies have been performed to find coating materials that are resistant to infiltration by CMAS. Notable examples are commonly-assigned U.S. Pat. Nos. 5,660,885, 5,683,825, 5,871,820, 5,914,189, 6,465,090, 6,627,323, 6,720,038 and 6,890,668, and U.S. Published Patent Application No. 2007/0116883. The protective coatings of these documents can be generally described as being impermeable, sacrificial, or non-wetting to CMAS. Impermeable coatings physically inhibit infiltration of molten CMAS, sacrificial coatings chemically inhibit infiltration by reacting with CMAS to increase its melting temperature and/or viscosity, and non-wetting coatings reduce the attraction between the solid TBC and the molten CMAS.
Notwithstanding the above advancements in coating technology, the ability to detect CMAS on TBC's of turbomachine components would be very desirable. However, a complication is that confirmation of the presence of CMAS requires chemical analysis, which using conventional equipment would require removal of the component from the turbine. Consequently, CMAS detection has typically been performed during regular maintenance schedules and based largely on visual observations. Accordingly there is an ongoing need for more convenient and less obtrusive techniques to detect the presence of CMAS. It would also be desirable if the capability existed to predict the useful life of the TBC and the component it protects based on the presence of CMAS.